mtd: spi-nor: spansion: Add local function to discover page size

The page size check in s28hs512t fixup can be used for s25hs/hl-t as well.
Move that to a newly created local function.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220725092505.446315-7-tudor.ambarus@microchip.com